Production of Novel Textile-Based Artifi cial Anterior Cruciate Ligament

The aim of this research was to produce artifi cial anterior cruciate ligaments (ACL) using the weaving (plain and leno) and braiding techniques, and to compare the mechanical and fatigue life properties of these ligaments with the natural ACL. For this purpose, tubular fabric structures were produced on braiding and weaving machines from polyester and Vectran yarns. To turn these structures into ligaments, the core of the tubes were fi lled with straight yarns. The mechanical properties of the resulted ligaments were tested before and after a fatigue test. The results showed that all produced ligaments provide enough tensile strength and breaking elongation when they were compared with the natural ACL mechanical properties. After the fatigue test, the tensile strength of ligaments did not decline substantially. Regarding the tensile strength, the leno weaving structure has the most similar properties to the natural ACL. The leno weaving ligament made from Vectran has one-third of the strain percentage of the natural ACL.


Introduction
A ligament is a band of tough, fl exible fi brous tissue that connects bones in the body to other bones and keeps these structures stable.Th e human body has 900 ligaments, of which 10 are located in the knee joint.Th e anterior cruciate ligament (ACL) is one of the four major knee ligaments, which provide stability of the knee joint by controlling excessive knee motion.In other words, ACL restricts both the horizontal translation (anterior-posterior and medial lateral translation) and relative rotation (varus-valgus and internal-external rotation) between femur and tibia (Figure 1) [1][2][3].ACL might be subjected to overloading during heavy physical activities and sports, resulting in a sprain or tear of the ligament.An ACL tear is a very common sports injury.Th e injury rates are higher for women.Th e tearing happens during a rapid change in direction when moving, slowing down when running, landing aft er a jump or receiving an abrupt force to the knee.ACL has a poor healing ability, since it is enveloped by synovial fl uid and lacks signifi cant vascularization.In consequence, its treatment frequently requires surgical procedures.According to Legnani et al. [4], this reconstruction operation is performed to 100,000 patients per year in the USA, 3,000 per year in Sweden.Th ere are some options for a surgical treatment including auto graft s, allograft s and artifi cial graft s.In the auto graft ing operations, patient's own tissues (most commonly the middle third of the patella tendon and hamstring tendons) are used for the treatment.Auto graft s provide good results; however, they do not eliminate some risks like morbidity secondary knee pain, patellar tendonitis and fracture or hamstring tendon weakness.In allograft operations, cadaveric tissue coming from a tissue bank (human or animal tendons) is used.Th ere are numerous risks at allograft due to limited Cetin Aka 1 , Guldemet Basal donor tissue sources, disease transmission and tissue rejection [5].In artifi cial graft s, a natural ACL is mimicked by the structures made from synthetic fi bres.Artifi cial ligaments are used to replace the ruptured ACL, to support tissue growth, to take applied loads until the new tissue grows and to shorten the healing period.Today, despite some disadvantages such as synovitis, a synthetic biocompatible material based artifi cial ligaments such as Carbon, Gore-Tex, Dacron, Leeds-Keio, Kennedy LAD (ligament augmentation devices), Stryker, Ligastic-Ligapro and LARS (ligament advanced reconstruction system) are implanted to injured ACL patients [4][5][6].[3] Th e aim of this research was to produce artifi cial anterior cruciate ligaments (ACL) using the weaving and braiding techniques, and to compare the mechanical properties of these ligaments with the natural ACL before and aft er a fatigue test.

Experimental
As test materials, artifi cial ACL were produced from high tenacity fi lament polyester (1100/192 dtex) and Vectran (220 dtex) staple yarns.To mimic the natural ACL, tubular structures were produced on an automatic sampling loom: Model SL8900s, Kus narrow weaving machine (similar to Jakob Muller needle loom) and 48 carriers circular braiding machine.Th e samples woven on an automatic sampling loom had a leno weave structure and the samples woven on a narrow weaving machine had a plain weave structure.Th e leno weaving and braiding structures were chosen to provide resistance against abrupt and directional forces.Following the production of tubular fabrics, core parts of these fabrics were fi lled with straight polyester and Vectran yarns to construct artifi cial ligaments.Th e objective of this bicomponent structure was to increase tenacity against high forces and facilitate cell adhesion on the surface when it is implanted.Table 1 summarises the production methods and components of ligaments.Th e mechanical properties of ligament samples were measured on a ZWICK Z10 test machine.Th e test samples were conditioned in a standard atmosphere for minimum 24 hours.Tensile tests were conducted by using a constant speed gradient dynamometer at the speed of 10 mm/min [3] with test length of 40 mm [3], and tenacity and breaking elongation values were recorded.To simulate the actual wear condition, the textile woven based artifi cial ligaments were subjected to a fatigue test using a dynamic fatigue tester which is very similar to the one developed by Marzougui et al. and Jedda et al. [7,3].Th is fatigue tester generates cyclic movements, i.e. bending, twisting and traction, synchronously to mimic the knee motion during the walking.Th ese deformations are performed in a saline medium at 37 ± 0.5 °C [8,3] to simulate the synovial liquid in the knee joint.In our test, to analyse the creep properties and to predict the lifetime of the ligament, a fatigue test was performed at 1.66 Hz, which is the actual knee motion frequency during the walking [9,3].Th e motion angles were 60° and 15° [7,3].Since fl exion is related to the inclination angle of the tunnel from the Tibial plateau for artifi cial ligaments, they are subjected to the deviation angle 60° at the fl exion test [3].Th e bending angle chosen for our study was thus 60°.Aft er 50,000 ± 100 of fatigue cycles, the tensile properties of ligament samples were measured again and compared with the untreated samples.

Results and Discussion
Th e mechanical properties of the resultant artificial ligaments, e.g.tenacity and breaking elongation, were revealed through the mechanical tests and the results were compared with the natural anterior cruciate ligament (Figure 2).   Figure 2 shows that the material and textile structure play an active role in tensile strength and breaking elongation.When considering the fact that the maximum tensile strength of the natural ACL of a middle-aged man is approximately 2,160 N, it is clear that almost all produced ligaments provided suffi cient tensile strength.Only sample 6 had slightly lower tensile strength compared to the natural ACL.As for the elongation at break, the narrow woven ligament made of PET and fi lled Vectran yarn (sample 5) had the lowest elongation value.It is likely that the Vectran core restricted elongation.As expected, the braided samples had the highest elongation values (samples 3-4), since braiding provides a more fl exible textile structure.Technically, when the force is applied to a circular braiding structure along the longitudinal axis, yarns slip easily and the braiding angle increases, since the yarns of the braided structure are crossed diagonally.Aft er 50,000 ± 100 dynamic fatigue cycles, the tensile properties of ligament samples were measured again and compared with the untreated samples.Figure 3 compares the tensile properties of ligaments before and aft er the fatigue test.Th e results showed that the textile structure infl uenced the fatigue properties of ligaments.In general, as expected, tenacity of ligaments decreased aft er the fatigue test.Only in sample 2, there was a slight increase in tensile strength, but this was more likely due to an experimental error.Apart from sample 2, all ligaments showed similar fatigue properties.Th eir tensile strengths did not decrease substantially.Figure 5 shows the maximum tensile strain of ligaments before and aft er the fatigue test.Samples 3 and 4 had the highest elongation percentage, which might be related to the braid structures of samples 3 and 4 having more fl exibility than the others.Th e breaking strain of the leno ligament made from Vectran (sample 5) equalled one third of that of a natural ACL before the fatigue test, while aft er the fatigue test, it equalled one half of that of a natural ACL.Nevertheless, it is expected that breaking strain would be lower aft er the fatigue test as due to permanent deformation, the breaking strain values of samples 1 and 3 remained the same.It can be said that samples 1 and 3 had higher resistance against the fatigue cyclic motions (bending twist, traction etc).Comparing the strain percentage, it can be seen that the leno weaving ligaments made from Vectran have the lowest strain value.All ligaments, except for number 4, had similar elongation properties with a natural ACL.It seems that there was a slight decrease in the strain value aft er the fatigue cycles for sample 4, but since this might be related to an experimental error, the tests will be repeated.It was expected that the value of the breaking strain of all ligaments would decrease aft er the fatigue tests when they were compared with virgin ligaments.Th e reason for obtaining opposite results is that the tests were performed in saline media at 37 °C.

Conclusion
Th e aim of this research was to produce artifi cial anterior cruciate ligaments (ACL) using the weaving and braiding techniques, and to compare the mechanical properties and fatigue behaviour of these ligaments with the natural ACL.Th e results showed that almost all produced artifi cial ligaments had higher strength values than the natural ACL.With regard to the elongation at break, the resultant ligaments had either higher values or lower values compared to the natural ACL.Th e tensile strength values of the ligaments did not change aft er the fatigue test.Th e closest value was obtained through the leno weave ligament made from Vectran compared to the natural ACL.In addition, this ligament showed higher resistance against dynamic fatigue behaviour.In conclusion, some encouraging results were obtained with this study.However, to determine the fatigue lifetime of ligaments, the number of test cycles should be increased.

Figure 3 :
Figure 3: Tensile strength of ligaments before and after fatigue test (50,000 cycles)

Figure 5 :
Figure 5: Maximum tensile strain of unfatigued ligaments and fatigued ligaments (50,000 cycles) 2 1 Dokuz Eylül University, Faculty of Engineering, Department of Textile Engineering, Tınaztepe Campus 35397 Buca, Izmir, Turkey 2 Ege University, Faculty of Engineering, Department of Textile Engineering, 35100 Bornova, Izmir, Turkey Th e braiding diameter consequently decreases and the braid length increases.Th e leno structure has weft and warp yarns like a conventional weaving structure.However, in leno weave, yarns are locked in place by crossing two or more warp yarns over each other, interlacing with one or more weft yarns.When the force is applied on the leno structure longitudinally, warp yarns start to squeeze weft yarns, cutting them eventually.Low tensile strength can be associated with the ligaments with the leno structure.

Table 1 :
Produced artifi cial ligaments and their component properties